Electron emissive electrode



M. E. MACKSOUD 2,916,653

ELECTRON EMISSIVE ELECTRODE Filed April 1, 1957 Dec. 8, 1959 INVENTOR. M/cfifz Z'T MAC/(500D BY QM Q A TTOIPNEVS' United States Patent ELECTRON EMISSIVE ELECTRODE Michel E.. Macksoud, New York, N.Y., assignor to Burn- Test Corporation, North Bergen, N.J., a corporation of New York Application April 1, 1957, Serial No. 649,987

' 1 Claim. ((11. 313-346 I I my copending application Serial No'.-570,757, filed March 12, 1956 for Electron 'Emissive Assembly, now abandoned. i v IBetter operation isobtained by the structure herein disclosed in that it provides an increase in-theionization of 7 the "inert-gas such as argon and the mercury content of discharge'devices of this type, that is in'the greatly increased production of ions. Another object of this invention is to provide an elec trode assembly which by its construction insures that the arc discharge associated therewith will be guided to and become stabilized at the center of the assembly.

Still another object is to provide an electrode assembly in which the parts are mechanicallyunited without requiring welding them together.

"f Other and more detailed ob ects of the invention will be'iapparent fromthe following disclosure of the embodiment thereof illustrated in the accompanying drawings.

In thedrawings, t V

)Eigure 1 is a topplan view of the electrode assembly offthis invention'showing a portion" of one end' of the associated envelope mostof whichhas 'been broken away;

Figure 2 is a side elevational view of the structure of Figure 1 with the envelope portion in cross section and a part of the structure broken away for better illustration;

Figure 3 is a detailed view of an element of the assembly; and

Figure 4 is a greatly enlarged detail view with parts broken away of the upper end of the assembly.

The electrode of this invention is adapted for use in all types of electron discharge devices depending upon for example, be of tungsten. The envelope and its stems are preferably made of quartz, glass or other equivalent efficient transmitter of ultra violet energy. As shown the lower ends of the rods 14 and 18 are flattened and have welded or otherwise attached to their fiat faces the lead in connections '16 and-20 respectively which are preferably of ribbon form in the region of which the main seal against the entry of air from the outside into the envelope is prevented.

A supporting rod 18 projects into the envelope and has mounted thereon a tightly wound coil 22 of tungsten or other equivalent metal wire for the purpose, which is either thoriated or coated with an electron emissive coating, "or both, as is well'understood in the art. The coil 22 provides a substantially "continuous sleeve or sheath mounted on the projecting end of the support rod 14, so that the free end of that rod projects beyond the end of the coil 2'2 ashort distance. The coil 22 can take many forms and is also disclosed in my copending application Serial No. 542,317, filed October 24, 1955 for Electron'Emissi've Electrode and Method of Making, now abandoned. The coil 22 isin physical contact with the rod 14 to provide electrical and heat interchange between them, and is notattached thereto by 'Welding.

- Surrounding and coaxially related to the coil 22 is a refractory metal coil 24 of tungsten wire, for example,

which is of such radial dimensions as to be spaced from the surface of the coil 22. i As shown the convolutions of the coil 24 are preferably spaced at least the thickness of the diameter of the wire of which'they are composed.

' As illustrated in the drawings, the wire'of the coil 24 is end .32 of the rod 14 so as to not require welding tothe creation of an arc discharge in an ionizable atmosphere for its operation. For purposes of description the utility and functioning of the electrodes will be well understood by relating it to the operation of an are discharge lamp rich in the production of ultraviolet light.

As shown in the drawings, an arc discharge lamp of this type includes an evacuated envelope, a portion of which is shown at 10, having an integral stem 12. The construction shown in Figure 2 to complete the device will be duplicated at another point on the envelope, usually at the other end, so that both assemblies are aligned on the axis'of the envelope. As is usual, the envelope will be highly evacuated and provided with an ionizable atmosphere which commonly includes one of the rare gases, as for example argon and a small quantity of mercury.

Sealed in the stem 12 are a pair of refractory metal rods 14 and 18 which are sealed in the stem and can,

geth'er Thecoil 24 is designed tohave a'resistance to provide' a sufficient voltage drop across the connection between the small turns 30 of the coil 24 and the point of connection of the lead 26 to the rod 18. This voltage drop is of such magnitude as to cause, upon the passage of current through the coil 24, substantial ionization of the rare gas content of the envelope 10, and vaporization of its mercury content. Such ionization is facilitated by the heat generated in coil 24 and by electron bombardment of the lower end of the coil 22 by the existence of a maximum potential difference at that point. In addition the electron emissive member 22 is also heated by radiation from the coil 24 as well as by electron hombardment so that it too begins quickly to emit electrons during the starting of the discharge device. This insures that the main discharge between a pair of such electrons and sucha device is quickly initiated.

As is well understood in this art the coil 24 is normally included in an auxiliary circuit containing a thermostatic switch so that that circuit is interrupted shortly after initiation of the main arc discharge between the electrode assemblies.

One of the advantages of this arrangement is minimization of the tendency of the device to blacken its envelope opposite the electrodes by reason of the fact that the main arc discharge starts after heating of the emissive surfaces 22. In other words, the arc does not start while the electrode assemblies are cold, as is the case with other electrodes heretofore used in devices of this kind.

The above described subject matter is substantially disclosed in my above mentioned copending application. The structure herein is characterized and distinguished therefrom by provision of a helically wrapped wire 28 applied to the first turn of the coil 24, as shown in Figures l, 2 and 3. The wire 28 is of a refractory metal such as tungsten and is preferably thoriated or coated with an electron emissive coating, or both. Such coating can, for example consist of a slurry of 2% thoriated tungsten and barium azide which after application is fired in a vacuum to produce an alloy of thorium and barium metal in the form of a coating thereon. This insures copious electron emission from that coating. The wire 28 is wrapped around the wire of the last portion of the coil 24 so as to effect tight contact therebetween, insuring physical fixing of the turns 25 on the last turn of the coil 24. As illustrated, the turns 28 are contiguous thereby increasing the overall mass of the coil 24 in the wrapped area. As shown in Figure 1, the turns 28 terminate at the point of beginning of the coiled end 30 of the coil 24.

In the operation of this device when the arc is first initiated it tends to strike towards the lowermost portions of the coil 24 as at A, and continues to move axially along that coil through the points B, C and D until it reaches the first of the turns 28. It then progresses around the last turn of the coil 24 through the points E, F, G, H and 1, until it reaches the end 32 of the support rod 14 where it stabilizes. The highly emissive nature of the wire 28 wrapped around the last turn of the coil 24 and the irregular surface presented by it has the property of urging the arc discharge to be guided to the .end 32 of the support 24 where it is desirable that it stabilize. for the most eflicient operation of an arc discharge the arc should be more or less symmetrical about the longitudinal axis of .the envelope, that is the axis common to the two spaced electrodes.

It will be understood that the subject matter of this invention is not necessarily limited to the application of the wire '28 to the last turn only of the coil 24 since useful results can be secured by wrapping wire 10, 28 around additional turns and in some cases of the turns of coil 24.

In the modified structure of Figure 4 the only diiference is that the turns of the small coil 30 at the upper end of the coil 24-are also wrapped with the wire-28 in addition to still further contribute to the arrival of the are at the end 32 of the support 14. These additional wrappings of the wire 28 provide a mechanical aid in facilitating the application of the coil to the rod and the anchoring of it thereon. With the turns 30 properly dimensioned and with the wire '28 wrapped around the convolutions thereof, the coil 24 can he slipped onto the end of the support 14 and thereby locked in place.

In both forms of this device it is desirable that the end 32 of the support 14 project but a short distance beyond the turns 30, as for example a distance of the order of one to five millimeters.

The electron emissive efliciency of the coil 22 can be increased by coating it with a thorium or other emissive coating which may be underlaid in some cases with a platinum coating, which may be plated thereon. The underlay of platinum reduces the work function of the electrode, that is causes it to have a higher electron emissivity at a given operating temperature.

Those skilled in the art will appreciate from the above. description that the subject matter of this invention is capable of some variation and it is dmired therefore that the disclosure be taken in an illustrative sense and that the scope of protection afforded hereby be determined It is, of course, appreciated in this art that by the appended claim.

What is claimed is: e

. An electrode for electron discharge devices comprising an electron emissive member, a heating coil spaced from and surrounding said member, a conductive connection between one end of said member and said coil, a refractory wire wrapped around at least the lastturn of-said coil adjacent said connected end and current input leads connected to said' member and the other, end of said coil, said wire having a coating comprising an alloy of thorium and barium.

A s n 7-; Aug- ,1 54 

